Photothermographic materials which are processed by a thermographic process to form photographic images are disclosed, for example, in U.S. Pat. Nos. 3,152,904 and 3,457,075, D. Morgan and B. Shely, "Thermally Processed Silver Systems" in "Imaging Processes and Materials," Neblette, 8th Ed., Sturge, V. Walworth and A. Shepp Ed., page 2, 1969.
These photothermographic materials generally contain a reducible silver source (e.g., organic silver salt), a catalytic amount of a photocatalyst (e.g., silver halide), a toner for controlling the tone of silver, and a reducing agent, typically dispersed in an organic binder matrix. Photothermographic materials are stable at room temperature. When they are heated at an elevated temperature (e.g., 80.degree. C. or higher) after exposure, redox reaction takes place between the reducible silver source (functioning as an oxidizing agent) and the reducing agent to form silver. This redox reaction is promoted by the catalysis of a latent image produced by exposure. Silver formed by reaction of the organic silver salt in exposed regions provides black images in contrast to unexposed regions, forming an image.
Recently in the medical field, the imaging technology has made an advance with a focus on the keyword of easy, rapid and Earth-friendly processing. For example, the development of a medical image recording system which is laser exposed and heat developed started, and a dry system discharging a processing solution as little as possible has become widespread. Meanwhile, the laser output means has also made a progress to offer a wider range of choice of lasers, and the demand for visible light lasers is increasing.
For such photographic photosensitive materials, there are known in the art various methods of containing dyestuffs in the photosensitive materials to adjust the tone thereof in order that an image as a whole look in a more black tone, for example, a method of using oil-soluble dyestuffs in photographic silver halide photosensitive materials as described in JP-A 243654/1985, a method of using an emulsified dispersion of specific anthraquinone dyestuffs as described in JP-A 139607/1989, and a method of emulsifying and dispersing a colored dyestuff together with a polymer and introducing the dispersion into an emulsion layer as described in JP-A 289227/1993 and 341441/1993.
However, when dyestuffs are contained in emulsion layers of photothermographic materials by such prior art methods, the dyestuffs can adversely affect photographic properties by increasing the fog by development and reducing the sensitivity beyond the necessity.
Moreover, due to the co-presence of polyhalogenated compounds important as antifoggants for heat development, the dyestuffs can be bleached during storage whereby the image tone is deteriorated or discoloration occurs.